The present invention relates to an open cycle, internal combustion, Stirling engine. More particularly, the present invention relates to an open cycle, internal combustion, Stirling engine of improved operating characteristics enabled by the combination of various means including specially configured manifolds and various valve mechanisms for controlling the flow of the air or other working fluid.
The Stirling or "hot gas" engine is a type of engine long known in the art. Although greatly overshadowed in commercial use by engines using the Otto or Diesel cycles, the Stirling engine has long been the subject of study because of several practical and theoretical advantages it has over more common engines.
The Stirling engine bases its design in an attempt to simulate the Stirling thermodynamic cycle. The Stirling cycle is, in theory, a cycle comprising constant volume and constant temperature processes which has theoretical efficiencies much higher than those found in the Otto or Diesel cycles. However, the Stirling cycle has had difficulty in finding practical applications because of design barriers in manufacturing an engine capable of efficiently and quickly performing the constant volume and constant temperature steps in the cycle. For a more elaborate history of the Stirling engine, see; Walker, Stirling Engines, Oxford University Press (1980) which is incorporated herein in its entirety for all purposes.
Among the various alternatives for Stirling engine designs, most designs employ "external" combustion, typically in a closed cycle engine, as shown in U.S. Pat. Nos. 3,442,079 and 3,399,526. The combustion is labeled "external" because the combustion occurs outside or separate from the working fluid and the heat generated is transferred to the working fluid. This differs from the more common internal combustion engine where the working fluid (i.e., air) is mixed with fuel and ignited.
External combustion, closed cycle Stirling engines have a number of practical and technical drawbacks. First, the working fluid normally chosen for a closed cycle engine is hydrogen or helium instead of air because of the higher power density and higher thermal efficiency offered by those substances compared to air. However, the use of hydrogen or helium at pressures above 100 bar may produce significant problems. For instance, safety hazards from explosions are present when hydrogen gas is used, especially under high pressure. Also, limitations exist in the selection of materials, particularly seals and rings, which can function under these conditions, because it is necessary to store the working fluid without allowing it to escape outside of the engine.
Further, because Stirling engines typically operate on a closed cycle, the heat transfer which must occur during the constant temperature steps is normally required to occur through an impermeable wall so that the enclosed working fluid does not escape. The drawback to this design requirement is that efficient and rapid heat transfer through the impermeable wall may normally be gained only by using large surface areas in the wall, which increases weight and expense, and reduces efficiency. The amount of heat transferred through the wall is represented by: EQU Q=A.times.h.times..DELTA.T
Where A is the surface area of the wall, h is the overall heat transfer coefficient of the wall, .DELTA. T is the temperature difference between the opposite surfaces of the wall and Q is the heat transferred. Because Q and h are relatively fixed in an engine (the choice of materials typically being limited by economic considerations), an increase in the surface area A implies a lower temperature difference .DELTA. T, which increases efficiency. However, increased surface area A creates excessive dead volume in the engine, which decreases efficiency. As a result, closed cycle Stirling engines tend to have higher dead volumes than are desirable.
An open cycle, internal combustion Stirling engine would avoid many of the problems of a closed cycle, external combustion Stirling engine. First, the working fluid of an internal combustion engine (normally air) is stable when compared to hydrogen. Also, air is freely available, which minimizes the sealing and storage problems. Other advantages in engines of that design, such as faster start up time and more rapid acceleration capability, have been recognized in U.S. Pat. No. 4,004,421, issued to Cowans. As far as known, that patent appears to be one of the first disclosures of a design for an internal combustion, open cycle Stirling engine.
However, the engine disclosed in Cowans is believed to be unduly complex and appears costly to produce. Cowans discloses a compressor-expander system connected to an additional regenerator for intake and exhaust of the working fluid. The compressor-expander system pressurizes and depressurizes the working fluid before introducing it into the hot and cold chambers, thus raising the mean effective pressure in the cylinder. The Cowans design would, therefore, more properly be described as a "semi-closed" cycle engine rather than as an "open cycle engine". Obviously, the compressor-expander system adds expense to the cost of manufacture and reduces power output from the engine because of the power needs of the compressor-expander system. Other U.S. Patents which may relate to the present invention are U.S. Pat. No. 3,638,420, issued to Kelly et al; U.S. Pat. No. 2,951,334, issued to Meijer; and U.S. Pat. No. 3,180,078, issued to Liston. Each of those patents discloses a particular closed-cycle Stirling engine in combination with various heat transferring means and relative configurations of pistons. Although not referring to open-cycle, internal combustion Stirling engines, other internal combustion engines are disclosed in the following U.S. Patents and may have preceded the present invention: U.S. Pat. No. 4,344,405 issued to Zaharis; U.S. Pat. No. 1,372,216 issued to Casaday; U.S. Pat. No. 1,512,573 issued to Breguet; U.S. Pat. No. 3,177,856 issued to Perkins; U.S. Pat. No. 2,091,410 issued to Mallory; U.S. Pat. No. 4,114,567 issued to Burton; and U.S. Pat. No. 4,011,839 issued to Pfefferle.
The present invention improves on Stirling cycle engines known in the prior art in many ways, including by the employment of an intake and exhaust system controlled and enhanced by standard engine valves. The present invention also does not require a complex, costly compressor-expander system and therefore greatly simplifies the design and operation of the engine. Many other advantages and improvements of the present invention will be evident from the following descriptions.
Therefore, it is an object of the present invention to provide an improved open cycle, internal combustion Stirling engine. It is also an object of the present invention to provide an improved open cycle, internal combustion Stirling engine which is simple in design and efficient in operation.
These and other objects, features and advantages of the invention will become evident in light of the following detailed description, viewed in conjunction with the referenced drawings and appended claims, of an open cycle, internal combustion Stirling engine according to the invention. The foregoing and following description of the invention is for exemplary purposes only.